The present invention relates to a sheet feeding unit for sheet rotary printing machines.
More particularly, it relates to a sheet feeding unit for sheet rotary printing machines which selectively is used for face printing or face and back printing, and in which the sheets during the face and back printing can be turned in accordance with the principle of the sheet rear edge turning, and in which devices are arranged under a printing cylinder located before a turning cylinder, to conduct and to guide the sheet on the front edge in face and back printing during the turning step.
Such devices for guiding and conducting of the sheet front edge which after the sheet turning in the face and back printing becomes a rear edge are known, for example from the German Pat. No. DD-PS 101,336. This reference shows in FIG. 1 a guiding device arranged underneath the printing cylinder and including a gripper system which performs one revolution. The gripper system is formed as a cam-driven double rocker with the gripper arranged on the connecting rod. A second gripper system is provided on the printing cylinder. This gripper system includes a cam-driven double rocker with the gripper abutment articulately connected to the connecting rod. In addition, a guiding passage is provided, which is composed of two conducting elements having a constant distance from one another. Air can be blown in the guiding passage through conductor elements to carry the sheets.
The above described device has the disadvantage in that it involves high technical expenses resulting from two auxiliary gripper systems as well as the guidance of the sheet end on a connecting rod cam. By guiding the sheet end on a connecting rod curve it is moved discontinuously. Thereby it can lead to compressions of the sheet and therefore to its contact with the conducting element. This contact can lead to a smudging of the printed downwardly facing surfaces.